Retrograde Orbits

You'd hardly credit it. The single-minded and rather obscure obsession of a few astro‑anoraks led to the development of the modern world. Unbelievable? Read on.

For thousands of years the path of the planets across the night sky puzzled astronomers greatly. Most of the time these planetary orbits made their way from east to west, in just the same way as the fixed stars behind them. (Okay, so they're not really fixed, but they're so incredibly far away they might as well be).

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Now, the very word planet means 'wanderer', a good indication of their annoying habit of turning around and moving back the way they came. Ancient astronomers were convinced the universe was Earth-centric. They were also convinced that everything in the heavens moved about the central Earth in circular orbits, at regular speed. But if this were true, how could one possibly explain the peculiar dance of the planets?

Great stratagems were dreamt up to provide an answer. The Greeks drew up elaborate schemes that saw a system of wheels within wheels, the planets grinding around the Earth. And the Earth was the centre of everything: the centre of motion, the centre of their gods' cosmos, the centre of creation. And since there could only be one centre, or so the argument went, that meant there was no life out in space. There was no room for the alien.

It would be wrong to think that absolutely all the Greek thinkers held this notion. There was one philosopher, Aristarchus, who got it right. Somehow, more than two centuries before Christ, Aristarchus worked out that we lived on a moving platform, called the Earth, in a Sun-centred solar system. Naturally, nobody believed him. Well, you can't blame them really, can you? All the apparent evidence was stacked against it. Night after night, Moon, stars, planets (and fuzzy objects) seemed to revolve from east to west, without fail. Listen: it's worth taking a look at the night sky with this in mind. And it's worth thinking about this too: prove, with only the knowledge of the naked-eye astronomy known to the ancient Greeks, that the universe is Sun-centred and not Earth-centred, as most believed. It's not easy.

Despite its profound wrong-headedness, the Earth-centred system held sway for thousands of years. Salty sea-captains from Portugal and Spain used their skills in astronomy to navigate the oceans in the name of piracy and plunder. By the time the British had joined in the search for booty, a Polish astronomer named Nicolas Copernicus had renewed the old-age obsession with the planetary orbits.